1. Field of the Invention
This invention relates to pedal-type drive mechanisms for vehicles such as bicycles and more particularly to vehicle drive mechanisms for converting oscillatory motion of pedal levers to intermittent unidirectional rotation of a driven wheel and including means for continuously varying the speed of rotation of the driven wheel relative to the angular rate of movement of the pedal levers.
2. Description of the Prior Art
The conventional roller chain and sprocket bicycle transmission, driven by a pair of rotary pedal cranks, was designed a century ago and has remained essentially unchanged since then except for the development of various braking and shifting-type speed changing accessories. The basic arrangement of a forward sprocket wheel driven by rotating pedal cranks and coupled by an endless roller chain to a rear sprocket wheel mounted concentrically with the hub of the rear wheel of the bicycle combines the advantages of simplicity and low friction to provide an inexpensive and efficient drive mechanism. This conventional design has at least two inherent drawbacks, however.
For one thing, rotary pedal cranks do not provide a uniformly effective conversion of the substantially linear driving force exerted by the rider into drive torque. At the top and bottom of pedal travel, drive torque is minimal and reaches a maximum value only at the midpoint of downward travel of each pedal. For another thing, the endless chain and tandem sprocket drive does not lend itself to simple continuous speed change mechanisms. For example, two speed change systems in common use are the epicyclic or planetary gear changer developed by J. J. H. Sturmey and J. Archer in the early 1900's and the derailleur, which shifts the chain from one sprocket wheel to another of different size. The Sturmey-Archer type of epicyclic gear changer is very complex and relatively fragile and difficult to maintain in adjustment. The derailleur is a simpler mechanism but requires skill to operate. In addition, since the derailleur mechanism is necessarily mounted in an exposed position next to the rear wheel hub, it is subject to damage from dirt and impact.
Other drive mechanisms for bicycles or similar pedal driven vehicles have been used. One of the earliest bicycle drives, for example, was designed by K. Macmillan about 1839. Macmillan's Hobbyhorse, as it was called, had a pair of pedal levers pivotally suspended from the steering fork tube. Connecting rods extended rearwardly to the ends of crank arms attached to each side of the rear wheel hub, thereby converting oscillatory motion of the pedal levers into rotation of the rear wheel.
A drive mechanism disclosed in U.S. Pat. No. 2,252,156 issued to J. D. Bell on Aug. 12, 1941 combines the oscillating levers and connecting rods of the Macmillan design with a conventional tandem sprocket wheel and roller chain drive. The rear end of the connecting rod is coupled to the forward sprocket wheel through a complex slide and ratchet system for converting the reciprocal linear movement of the connecting rod into intermittent unidirectional rotation of the sprocket wheel.
The American "Star" racing machine, a well known bicycle of the last century, also featured oscillating pedal levers. The levers were pivoted from rear frame extensions behind the rear wheel hub and had pedals at their forward ends pumped in an up-and-down direction by a rider seated above the rear wheel. None of the above described oscillating pedal lever drives, however, had provision for changing the speed ratio, and none has survived in present day bicycle designs, although a slight variant of the Macmillan arrangement is used extensively for pedal drive mechanisms in four-wheeled vehicles for small children.